The hypothalamic suprachiasmatic nucleus (SCN) is the presumed site of the biological 24 hr clock of mammals. This clock is necessary for the timing of numerous behavioral and physiological rhythms. The proposed research seeks to describe 1) the roles of visual system components in the mediation of circadian rhythm phase, period and amplitude; 2) the anatomical connections between the geniculate region and the SCN; 3) the function and locations of SCN efferent fibers controlling circadian rhythmicity; 4) the relative roles of the paired SCN in regulating rhythmicity; and 5) the neuroanatomical substrate mediating phasic control of circadian periodicity by the benzodiazephine, triazolam. The latter drug offers a probe into the neural systems mediating circadian rhythm phase. These aims will be achieved using stereotaxic neural lesion techniques, drug infusions into the brain and modern neuroanatomical tract tracing methods. Locomotor rhythms will be continuously evaluated by computer. Because of the wealth of information concerning hamster circadian rhythms and its endocrine responses to different photoperiods, it is the species of choice for this work. All vertebrate species, including humans, have a circadian rhythm system. It is important, and clinically relevant to understand how and where environmental events regulate a fundamental biologic regulatory system.